Method of producing silicon.



Patented Dec. 29, 1908. 2 SHETS-SHEET lt H. N. POTTER.

METHOD OI' PRODUGNG SILIUON.

V APPLICATION FILED MAY 22, 90& 130.

x W ///U/////// y o WTNESSES:

H. N. POTTER.

METHOD OF PRODUCING SILIGON.

APPLIOATION FILED MAY 22, 1905.

Patented Dec. 29, 1908.

2 SEEETS-SEEET 2.

I NVENTO? flNEY W/ TNE SSE S.-

reduction of siL'con diox'd in' the umrete eren "s" rnrnnr 'erren HBNEYNOIELPOFTER, GF NEW? ROCEELLE, NEW YORK, ASSIGNOR TO GEO. WESTINGQUSE;

4 -F PETTS-BUBLG, PENNSYLVANIA.

METEO@ OJF' PRQDUCING SELECON.

Speccaton oi Letters Patent.

Patenta& Dec. 29, hfi

To all whom it may concerns Be it known that I HENRY NOEL Porrnn, ecitizen of the United States, and resident of New Rochelle, County ofW'estchester, State of New York, have i nvented certain new and usefulImprovements in Methods of Producing Silicon, of which the following isa s iecificetion. y

have invented a new and improved process 'of making silicon free fromsilicon carbd.

It is known that silicon can be produced by reducing silicon dioxid bymeans of cerbon in the free state or combined with silicon in the formof silicon cerbid.

The reactions heve been expressed as follows:

SiO, +20 Si 2CO(A) SiO +2SiO =3Si +2CO (B) It hes also been propo'sed toemploy reection A and keep the temperature below that necessary for theformation of SiC by the union of silicon and cerbon, or the resence ofexcess cerbon. Such e process is efective because silicon carbid isproduced at such low temperature as -to preclude any comrncrciellyefhcient yield of silicon, if indeed any at all, et temperatures stilllower. There is no evidence to show that anorphous silicon cerbidrequires any higher tempereture for its formation then silicon itself.The reection does not take place as stated in A, but in a mannerexpressible es follows:

( (M H), (2a)C (r) SiC +(s) Si +(t) SiO (2a-r)CO,

nezlecting the hypothetical (SiC)O or STOXCOD compounds.

` The above expression may be considered to be the simultaneousoccurrenoe of three reactions:

of the original (A') expression.

As e. n'uxture of granuler coke and send,

'i for example, is' made up of messes of enormous size relativetoindividual molecules, the mixture is not homogeneous like e solutionof one liquid in another, or' e nuxture of geses, but is e mass in whichet some spots cerbon is in excess, et others' silice, so that it isquite 'possible that the three reactions may go on simultaneously' asoutlned. There are four rincipal reaction products, SiC, Si, SiO end'.CO; which separate according to their properties. Si' dissolves some orall the Si@` and the two flow down till they either freeze or findlodgment before solidifyiig. The SiO end CO pass ofi` together, the Si@deposi'ting` in the gas vents through which CO escepes.

The dominant relaton is that there ere the same number of etoms of C esof O. If this balance be disturbed' in any way, for example, byvoletiliz-ing awey some' SO, mire duced, or by changing the rnixtureproportions in the beginning, the relative values of r, s and t will bechanged but experience shows that r cannot be made zero, in any mixtureyiel'ding a sufiicient quentty of silicon per kilowett hour to becomnercially e'tli'- cient. The reason ep cars to. be that 'fluid Siedheres firnly to S and dissolves et least. a portion of it.

The fluid silicon shields the conteined Si@ from rapid reaction With SO,which can react easily With the free carbon present. Increasng the SiOin the mixture doesnot prevent this, and does increase the value of t,representing energy diverted from Sll" con production and consequently areduced` yield of silicon.

The existence of the compound SiO'WaS discovered bythe applicent anddescribed in his. application #238,925, filed December Applicant is thusthe first to apprecete the presence and. importance of 810 eccompanyingthe production of silicon by manufacture of crystelline' sic', orcerborun dum, which contains e large amount of SC mostly in theemorphous state, *also some free eerbon and 510,. This crude. materialis called "cerborundum fire send and' vairies somewhat in composition, asample analysis showing:

`plicant's-analy`sis andltheir existence or nonex'stence is notimportant lIl the consderation ofthe reactonshere expressed. `llt isclear that with &lis firesand together with silica and extra coke ifdesired, a mixture. can be made having the composition,

(e) (o i i n c and under the conditions to be described, l

find that the largest yield of silicon is given.

by proportonng the mixture about as iollows:

(C') BSO -l-GSC +40 which may also' be expressed 3Si0 (B) 2Si0 +40 (A)ESiO showingit to be a combnation of the expressions (A) and (B) withexcess Si0 lt must not be assuned that the miXture stated yields Si freefroni SiC, as there is no'evidenee that it does, in fact it probablyyields an Si rich in SiC. What it does do is to yeld a large amount ofSi per kilowatt hour, 'which by a second ste in the process is freedfroni anycontained "iC.

Before describing the purificaton process I may say that it isapplicable to Si produced froni a inix'ture like expression (A) or like(B) or like (C'), the advantage of the latter being the larger yield perdollar eXpended on mixture power relative to (A) and (B) at presentprices.

Mixture formula (C') is by weight an analysis of an actual iniXtureshowed SiO L 59.72% Si\) 31.51% &00 Slag 276% This latter mixture run inan electric are furnace using alternating current, 32 K. W. on. priinaryduring 40 K. W. hours, ineasured at the are, yielded over 2000 gransol'available silicon, :1 rate ol over 50 gramsper K. Vi hour.

,iso

lt is possible to vary 'the relative proper tions 'of Si@ and C and touse slightly more or less eXcess SiO, but the best miXture proportion isreadily determined for any set of furnace conditions.

I will now describe my process of purification.

l have discovered that fluid silicon does not dissolve earbon, but doesdissolve silicon carbid. I have further found that SC and SiOfreactreadily heneath a bath of fluid silicon, the products being either Si,or SiO, .or both and CO. SiC can. also reaet with SO, the. productsbeing Si and CO. It is thus clear that if SO, be present beneath a bathof fluid Si, that there the SiO and contained SiC can react, While anySO liberated may react with SiC, so that the final roducts are Si, whichadds itself to the si icon already present, and CO, which esca es.

Even. though particles of carbon s iould he earried down into the bathmeehanieally, they can do no injury as, even assuming them to react Withthe bed and produee some SiC, this Would then be immediately removed byreaction with the bed. Under conditions of practice as described it isinconcevable that any quantity of carbon too great for the bed to takecare of can ever be introduced. My own belief is that no carbon whateveris 'brought down, and I further believe that the density of meltedsilicon is such' that carbon Will loat thereon, so that even ifwillfully introduced, it could 'not come in contact with the bed, exceptat the edges where it would constitute practically a part of themixture,

The temperature is sufiicient to cause a bed of granularquartz sand tofuse into a vitreous mass of sul'licient sustaining power to support thefused silicon in a hollow for ned in the silica bed.

I A. very satisfaotory arrangement is to use an electric are furnacehaving vertical coax- V ial electrodes, the lower surrounded near theare by' a bed of silica upon which surrounding the are and upperelectrode is a miXture like (C').

The mixture upon the bed must be deep enough to prevent the furnaceblowing which occurs when tl-e ese-a ing CO gas opens a large ventthrough -whchit escapes, carrying Sit?, SO and Si as dust or vapors withit, which is undesirable. V i

ll the CO escapes through a porous mass l of crushed carbor'for example,it is cooled enough to issue alone.

It is an advantage to have both electrodes movable, and. itis also anadvantage to.

operate long are at about 150 volts or higher, rather than done.diteulty of opera; oi a mass ol" nuxti e 4 i ,la uu.. Y 1 'm uppeiemetimin, llud at 1.00 volte, as can be i Lll freezing l about the 'erentirely preeoaieo venting any movement for changing the length oi tiarc during operation, and so difiicuit to remove after a run that manyeiectrodes have been broken. I find that this is satisfactorilv overcomeby incasing the upper electrode throughout the region where SiO iodgeswith slabs of carbon or granular carbon, or best of all, with slabssurrounded by granular carbon. The SiO deposits upon the slabs orgranules and the electrode remains free and after the run can be easilyremoved by cracking open the easng. The slabs are easily recovered andcan be used again.

A further advantage is to use a second siiica bed built in annular formaround the rpper eiectrode, the size and distance of this bed above thebottom bed depends upon the size of 'furnace and duration of run.

In operation there is a hollow formed about the .are and this hoiiowextends upward in peer shape and toward the end oi' the run reaches theupper silica bed which then dri s plastic siiica upon the lower bed andt e siiicon bath. The function oi this .upper bed se ms to be to form anarrower throat i'or the upwardiy growing hollow and thus consob tractthe region of reaction and in a measure compel the hollow to grow broadrather than tall, which is the tendency When uncontrolled. At any rate,the yield is improved per kilo watt hour by the upper bed, other thingsbeing equal. I have also tried entirely surrounding a miXture with anenvelop of silica, but this is a disadvantage, as the yield is reduced.This, 1 ascribe to the silica vi'trihiig into a dense mass whichconducts 'hoat much better than the reactivc niixture,

either when the latter is loose or fitted together into' a porous,slightly coherent mass.

I have tried a great number of 'ar-rangements of mixture zones,la`jy'ers, sectors, rings, etc., combined with other zonesdaycr,sectors, etc. of silica. Some of these arrangements aresatisiactory asto yield, and all are instructive in showing what probably occurs duringreacti'on, but the arrangenent described combines several desirablefeatures and is easily and chea lyconstructed.

While I have specied many details of a particular method of Operating myinvention, do not Wish to appear to state that there is but one suchmethod, and l: Will thereore describe a second method of operation.

In my former method, I have used as a source of heat an are betweencarbon electrodes. I have also tried furnaces of the resistance typehaving carbon resisters oi' oth continuous and granular type, but havefound them 'unsatisfactory, as the resistere are rapidly destroyed,apparcntly by the vapor oi silica, or silicon.

Another type of resistance furnace is to use the fluid silicon bath as aresistance and pass a current through it in any satis'factory marinar,as for example, by giving it an ex'tcnded shape between carbonelectrodes, preierably entirely immersed in the fluid silicon bath or insolid silicon extensons of the same, produced by keeping the temperatureof these extensions below the melting point of silico'n. Once formed,such a resistance bath can be kept at any suitable temperature andreactive rnxture added to it, tapping off the silicon produced, fromtime to time. Such a process may With advantage operate upon mixture ofsomewhat diiferentproportionsfrom that in the arc process, as there isno carbon introduced by electrode wear, etc. The distnctive feature ofreacting upon SiC by SO under fluid Si is however, characteristic ofthis process, as of the other.

teferring now to 'the drawings, Figure 1 shows a vertical sectionthrough the center of an arc furnace arranged substantially asdescribed; and Fig. 2 is a vertical section of a furnace after a comlete run showing the effect of the run and t e location or" the siiiconand silicon monoxid. The furnacc consists of a cast iron bottom plate i,1, supportcd on iegs 2, 2, and having a hoie 3, in its center. Oin thisbottom plate are sectional frames 4, 4, 5, 5, and 6, 6. These sectionalframes and the bottom plate are lined With fire brick 7, 7, 7, the brickon the bottom being provided with a hole 8, covered With an asbestosplate 9, through which projects ,the lower terminal 10, which is made ofgraphite. into the terminal 10, fits the electrode proper 11.. The uppereiectrode is composed of the round portions 12 and 13 and the squareportion 14 of larger size. Around the square portion 14 are arrangedcarbon slabs 15, 15, which in turn are surrounded by granular carbon 16,16. This ranular carbon toward the top is provide With a continuation*17. The furnacc about the electrodcs and within the framework is filledwith reactve miXture 18 18, and the various beds of silica 19, 20 and21. Bed 19 is the lower bed on which the silicon produced collects' andis purified as described. 20 is the annular bed, also described; 21 is acontinuation of 20, having a smailer central holc. The two togetherconstitute practically a conicai, hollow upper bed.

In operation a out 150 volts is applied to the arc and the upperclectrodc lifted, the arc becorning eventually between 4" and 5" inlength. The' u per portion-of the electrode 11 and the ower portion ofthe electrode 12 burn away. The portions 10 and 14 are not injured, theportion '13 "being but slightly injured' on its surface. After the runis over, the piece 13, with Whatever is left of 12, is unscrewed from14, and a fresh piece inserted between 14 and 13. In this way, theportions of the elcctrode injured &re replaced at the smallest expense.The seine principle of renewel can be applied to the lower electrode,but is not shown. The wear on the up er electrode considerebly exceedsthat ont e lower. I

Referring to Fig. 2, it will be observed that around the ere is ahollow, that the lower send bed 19, has become Vitrified above thedotted line 23,while uponthe vitried 'silice rests the silicon 24. Thehollow is surrounded by a o'ous, slightly cohering wall 25 25, where t ewall was forined from the mixturo 18. Above this in the neighborhood ofthe send beds 20 21, the surl ace is of vitrified silice 26 which hasdripped downward as indicated. Still further up is ,the mass,.-27composed of the miXture derived from '18, crushed carbon from 16, andsilicon monoxid which hes found lodgment in vents forme'd by the carbonmonoxid ges. Kfter the run, the furnece is taken down, being' sodesigned that this is aceomplished with very little labor, and the peershaped envelop removed, crecked open and the silicon obtained.

The wall mixture 25- and the SiO "mer teriel -27- are separated endsaved for making po\ 'dered SiO in a special furnace. The vitrifiedsjlica 23- etc. is saved for uses to which it is adapted, while themixture --1S-- can be run again. The granular carbon --16- is composedentirely of lumps which can be separated out by sifting the mixture 18-which passes through the sieve leaving the granuler cerbon thereon.

Cleims l. The process of elimineting silicon cerbid from silicon,consisting in reecting upon the said carbid by means of silicon dioxidunder fluid silcon.

2. The process of producing silico free from slicon cerbid, whichconsists in maintaining the silicon in a fluid state in contact Withsilicon dioxid, until all conteined oerbid is destroyed.

3. A process of purifying silicon f om SiC, consisting in maintaining itin e fluid state upon a bed of silica.

4. The method of producing silicon by the reduction of 'silica in thepresence of silicon cerbid, the collection of fluid silicon and anycontained silicon carbid upon a silice` bed and the oxidation of thecarbon of the contained silicon oarbid by reection with the said bed.

- 5. The method of pro'ducing silicon in en electric furnace by firstproducing a crude.

silicon containing silicon car-bid and by a second reacton n the senefurnace, oxdz mg the carbon of the contaned cer-bid under y fluidsilcon.

6. The 'process of producing silicon which consists n first reducng theshca of a shce eee, iso

, to flow down and collect upon e bed of slca and there causing reectonbetween the silicon cerbid end the said `bed, wherehy the seid carbid isbroken up.

8. A method of producing silioon which onsists in promoting e reactionbetween comminuted silica and silicon carbid, collecting the crude fluidsilcon. produced upon e bed of silice, and promoting rcection hetweenany silicon carbid conteined in said fluid silicon and the said silicabed until the silicon shell b'e freed from all contained cer bon. r

9.- The method of producing silicon which consists in piomoting ereuction between silicon (tai-bid and silica 'beneeth'fluid silicon inai: electric furn'ice which Supplies hoat for promoting the seidreection and also for producing crudc fluid silicon containing siliconcerbid, by a. rcection between comminuted silica and carboniferou-smaterial.

10. An 'electric furnecc process, yielding silicon and silicon nonoxid,which consists in ponioting reaction between silice and cerboniferousmaterial, in about the pr0portion of 8 noleculer equivelents of siliceto 10 utomic cqui'vnlcnts of .curhon.

11. An electric furnace process, yielding silicon and silicon nonoxid,Whichconsists in prornotinp reaction between silico and crboniferousmaterial in about the proper tion of 'eight moleculur equivztlents ofsince to ten ato nic equivalents of carbon, maintaining the collectingsilicon in a fluid state upon a bed ofsilice, and collecting the siliconnonoxid in the passeges travorsod hy the esceping gaseous i'c-actionproducts.

12. The method of 'producing silicon which consists in reducing silicebeneeth fluid silcon. i

13. The method of producing silicon which consists in electricallyfusing a both oi' silicon and promoting the reduction of silicethei-ein. y

14. The method of producing silicon, which consists in melting :tresistei: of silicon by means of elect'i'c current and eil'cctingtherein u reduction of silica Signed nt New York, in the county of NewYorl and State oi' New York this 15th day of May, A. l 1905.

HENRY NOEL POTTER.

VVitnesses: e

-`WM. H. CAPEL, GEORGE H. STocKBmDee.

